"In the early years of World War I, as many as 1,000 American horses per day were shipped off to Europe to assist in the Allied war effort, even though the United States was officially neutral. Those horses became the target of germ warfare, infected with anthrax cultures on American soil, while at the same time mysterious explosions were rocking U.S. munitions factories and fires were breaking out on ships headed to Europe.

"Journalist Howard Blum says this was all part of an aggressive campaign of spying and sabotage the German government unleashed on the United States soon after war broke out in Europe. Blum's book, Dark Invasion, is about the campaign and the effort of American law enforcement to crack what Blum calls 'the first terrorist cell in America.' It's filled with fascinating characters, from the duplicitous German ambassador who held the title of Count, to Captain Franz von Rintelen, who plotted destruction while living at the Yacht Club in New York, to the NYPD bomb squad detective who in effect formed an anti-terrorist squad to try and find the saboteurs.

"'There really wasn't anyone at first who could put it all together,' Blum tells Fresh Air's Dave Davies. 'Ships are blowing up at sea, or catching fire, factories are blowing up. Are these accidents? Are these industrial sabotage? No one really suspected a spy network.'"